1. Field of the Invention
The invention relates to a filter device, comprising a recursive filter unit, a transition detector for dectecting a transition in an input signal of the filter unit, and adaptation means for adapting the operation of the filter unit in response to detection of the transition.
The invention also relates to a transmission system which comprises a filter device and a receiver for such a transmission system.
The invention also relates to a method of filtering an input signal, in which the input signal is subjected to a recursire filtering operation and a transition in an input signal is detected, the recursive filtering operation being adapted in response to detection of the transition.
2. Description of the Related Art
Such a filter device and method are known from an article by V. K. Ingle and J. W. Woods, entitled "Multiple model recursive estimation of images", published in the Proceedings of the IEEE conference on Acoustics, Speech and Signal Processing 1979 (Washington, D.C.), pp. 642 to 645 (notably FIG. 2). This article deals with the problem of removal of noise from two-dimensional images such as video images.
Noise can be removed from an image by means of a low-pass filter. Such a filter, however, has the drawback that it blurrs the boundaries between the objects in the image. The cited Application proposes to address the problem by rendering the operation of the filter dependent on transitions in the image.
To this end, the filter comprises a gradient detector which associates each location in the image with one of five possible gradient classes:
(1) the image values in the vicinity of the location have a gradient-less structure, or PA1 (2-5) the image values are situated on a gradient which is classified in one of four directions (0, 45, 90 and 135 degrees, respectively).
The filter comprises a bank of five parallel recursive partial filters, each of which is suitable for the filtering of noise while maintaining sharpness for one of the classes. For each location, it is determined, in dependence on the detected class, which of these recursive filters has its output signal coupled to the output of the filter.
Lack of sharpness can thus be prevented, be it that five recursive filters are required. Circuits implementing the filter disclosed in the cited publication thus become complex. Moreover, the recursive filters need to average over mutually different, oriented regions, so that they deviate from one another and also have an individual complex construction.